msm: sde: Convert workq to kernel thread in SDE rotator

Fix performance issue in SDE rotator by coverting workq into kernel
thread. workq priority is not high enough and can be easily preempted.
For sde rotator being used for realtime deadline usecase, it is
necessary to move up the priority of the worker thread, and we need to
move into kernel thread to handle the works.

Change-Id: Id77e80cb69162326a3894a41bec295bb9bd7f5c1
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>